A sub-carrier multiplexed optical video signal transmission system employing a super wide-band frequency modulation scheme, in which sub-carrier multiplexed video signals are simultaneously converted into a single FM signal and a super wide-band FM signal thus obtained intensely modulates a transmitter laser, can suppress the effect of noise by wide-band improvement particular to FM transmission. Accordingly, the sub-carrier multiplexed optical video signal transmission system employing the super wide-band frequency modulation scheme is seen as a promising one providing a long transmission distance and permitting many splits, being especially suited for transmitting video signals, such as VSB-AM, for example, requiring extremely low noise. Concerning the optical sub-carrier multiplexed video signal transmission system employing super wide-band frequency modulation scheme, S. Suzuki and others presented a paper which is in the Autumn National Convention Record of IEICE of Japan, B-603, 4-63, 1991.
In the aforementioned FM based system, a white noise, which is superposed on a transmitted signal before the stage of demodulation, is converted into a particular noise, a rms value of which is proportional to frequency, and superposed on a demodulated signal at the stage of demodulation. In the aforementioned sub-carrier multiplexed optical video signal transmission system, the signal to noise ratio (SNR, hereinafter) of the sub-carrier decreases, as the frequency of the sub-carrier increases. Moreover, in the sub-carrier multiplexed optical video signal transmission system employing super wide-band frequency modulation scheme, it becomes necessary to modulate the frequency of a main carrier by the sub-carrier multiplexed video signals having an extremely wide frequency band. However, it is very difficult to provide a FM modulator having the extremely wide frequency band, hence, efficiency of modulation of the FM modulator inevitably decreases at frequencies near the highest end of the frequency band in many cases. Accordingly, the SNR of a sub-carrier with a higher sub-carrier frequency shows a tendency to decrease. Then, the transmission distance is limited to that of the sub-carrier channel with the lowest SNR, and it becomes difficult to increase the transmission distance.